MySQL多表SELECT

Question

我在组合MySQL查询时遇到了一些问题。它是多语言，多目标站点的一部分，因此我需要从多个表中检索站点的一些文本部分，并使用回退。可能命中的顺序：

  

更正@ local_tbl＆gt; english @ local_tbl＆gt;更正@ global_tbl＆gt;   英语@ global_tbl。

因为它会被高度使用，所以我希望保持快速且连接数少，但只返回1行。我尝试了以下几种方法，但我确信有更好的解决方案。

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FROM子句中的子查询：如果任何子查询给出0行，它就会死掉。即使它有效，也需要一些PHP解释：

SELECT * FROM

(SELECT `sp_content` AS sp_local FROM `loc-ae_siteparts` WHERE `sp_name` = 'name_of_some_sitepart' AND `sp_lang` = 'de' AND `sp_corrected`='1' LIMIT 1) as local,
(SELECT `sp_content` AS sp_local_en FROM `loc-ae_siteparts` WHERE `sp_name` = 'name_of_some_sitepart' AND `sp_lang` = 'en' LIMIT 1) as local_en,
(SELECT `sp_content` AS sp_global, `sp_corrected` as sp_global_corrected FROM `loc-global_siteparts` WHERE `sp_name` = 'name_of_some_sitepart' AND `sp_lang` = 'de' LIMIT 1) as global,
(SELECT `sp_content` AS sp_global_en FROM `loc-global_siteparts` WHERE `sp_name` = 'name_of_some_sitepart' AND `sp_lang` = 'en' LIMIT 1) as global_en

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TEMP表：这里我关注性能，不能使用内存引擎，因为它涉及文本字段。小鸟的核武器？

CREATE TEMPORARY TABLE IF NOT EXISTS `random_tbl_name` AS (SELECT `sp_content` FROM `loc-global_siteparts` LIMIT 0);

INSERT INTO `random_tbl_name` SELECT `sp_content` FROM `loc-ae_siteparts` WHERE `sp_name` = 'name_of_some_sitepart' AND `sp_lang` = 'de' AND `sp_corrected` = '1' LIMIT 1;
INSERT INTO `random_tbl_name` SELECT `sp_content` FROM `loc-ae_siteparts` WHERE `sp_name` = 'name_of_some_sitepart' AND `sp_lang` = 'en' LIMIT 1;
INSERT INTO `random_tbl_name` SELECT `sp_content` FROM `loc-global_siteparts` WHERE `sp_name` = 'name_of_some_sitepart' AND `sp_lang` = 'de' AND `sp_corrected` = '1' LIMIT 1;
INSERT INTO `random_tbl_name` SELECT `sp_content` FROM `loc-global_siteparts` WHERE `sp_name` = 'name_of_some_sitepart' AND `sp_lang` = 'en' LIMIT 1;

SELECT * FROM `random_tbl_name` LIMIT 1;

编辑：感谢所有答案，他们真的很有帮助。

Answer 1

SELECT * FROM
    ((SELECT 1 precedence, `sp_content`
     FROM `loc-ae_siteparts` 
     WHERE `sp_name` = 'name_of_some_sitepart' AND `sp_lang` = 'de' AND `sp_corrected`='1'
     LIMIT 1)
     UNION
     (SELECT 2 precedence, `sp_content` 
     FROM `loc-ae_siteparts` 
     WHERE `sp_name` = 'name_of_some_sitepart' AND `sp_lang` = 'en' 
     LIMIT 1)
     UNION
     ...
    ) x
ORDER BY precedence
LIMIT 1

Answer 2

当其中一个子查询为空时没有结果的原因是

select * from
s1, s2, s3, s4

你有一个隐含的联接。当其中一个子结果为空时，结果连接也是空的。

要解决此问题，您可以将子查询用作列。这将给出相同的结果，但是使用NULL值，其中subselect不返回任何行

SELECT (SELECT `sp_content` AS sp_local FROM `loc-ae_siteparts` WHERE `sp_name` = 'name_of_some_sitepart' AND `sp_lang` = 'de' AND `sp_corrected`='1' LIMIT 1) as local,
       (SELECT `sp_content` AS sp_local_en FROM `loc-ae_siteparts` WHERE `sp_name` = 'name_of_some_sitepart' AND `sp_lang` = 'en' LIMIT 1) as local_en,
       (SELECT `sp_content` AS sp_global, `sp_corrected` as sp_global_corrected FROM `loc-global_siteparts` WHERE `sp_name` = 'name_of_some_sitepart' AND `sp_lang` = 'de' LIMIT 1) as global,
       (SELECT `sp_content` AS sp_global_en FROM `loc-global_siteparts` WHERE `sp_name` = 'name_of_some_sitepart' AND `sp_lang` = 'en' LIMIT 1) as global_en

Answer 3

我过去做过类似的事情：

SELECT COALESCE(ls1.sp_content, ls2.sp_content) AS sp_content
FROM (SELECT 1) a
LEFT JOIN loc-ae_siteparts ls1
  ON ls1.sp_name = 'name_of_some_sitepart' AND ls1.sp_lang = 'de' AND ls1.sp_corrected = '1'
LEFT JOIN loc-ae_siteparts ls2
  ON ls2.sp_name = 'name_of_some_sitepart' and ls2.sp_lang = 'en'
LIMIT 1